The leaf of the lotus plant exhibits remarkable water-repellency and self-cleaning properties. Although lotus plants prefer to grow in muddy rivers and lakes, the leaves and flowers remain clean and are essentially non-wettable. The lotus plant achieves this effect by producing leaves and flowers with extremely hydrophobic surfaces. When the leaves come in contact with water, the water droplets contract into substantially spherical beads which roll off the surface, sweeping away any particles of dirt they encounter.
On extremely hydrophilic surfaces, a water droplet will completely spread and provide an effective contact angle of essentially 0°. This occurs for surfaces that have a large affinity for water, including materials that absorb water. On many hydrophilic surfaces, water droplets will exhibit contact angles of about 10° to about 30°. In contrast, on hydrophobic surfaces, which are incompatible with water, larger contact angles are observed, typically in the range of about 70° to about 90° and above. Some very hydrophobic materials, for example, Teflon™, which is widely regarded as a benchmark of hydrophobic surfaces, provides a contact angle with water of as high as 120°-130°.
Against this background, it is remarkable that the lotus leaf can produce a contact angle with water of about 160°, which is substantially more hydrophobic that Teflon™. The lotus leaf is thus an example of a “super-hydrophobic” surface. For the present purposes, a super-hydrophobic surface may be said to be one which provides a contact angle with water of greater than about 140°. This effect is believed to arise due to the three-dimensional surface structure of the leaf wherein wax crystals self-organize to provide roughness on a nano- or micro-meter scale. The hydrophobic surface protuberances reduce the effective surface contact area with water and thus prevent adhesion and spreading of the water over the leaf.
The discovery of the aforementioned properties of the lotus leaf and elucidation of its mechanism has led to a variety of engineered super-hydrophobic surfaces. Such super-hydrophobic surfaces have water contact angles ranging from 140° to nearly 180°. Such surfaces are extremely difficult to wet. On these surfaces, water droplets simply rest on the surface, without actually wetting to any significant extent. Superhydrophobic surfaces have been obtained in a variety of ways. Some of these very hydrophobic materials are found in nature. Other superhydrophobic materials are made synthetically, sometimes as mimics of natural materials.
U.S. Pat. No. 6,683,126 describes a coating composition for producing difficult to wet surfaces comprising a finely divided powder, where the particles are porous and have a hydrophobic surface, combined with a film forming binder such that the ratio of the powder to the binder is 1:4.
U.S. Pat. No. 6,852,389 describes the process of production of superhydrophobic materials for self cleaning applications.
U.S. Pat. No. 6,946,170 describes a self cleaning display device.
U.S. Pat. No. 7,056,845 describes a method for the application of a finishing layer which is water repellant for use in finishing of textiles, fabrics and tissues.
U.S. Pat. No. 6,800,354 describes process of production of self cleaning substrates of glass, ceramic, and plastics.
U.S. Pat. No. 5,500,216 describes a method of reducing drag through water by applying a film of rough particles of hydrophobic metal oxides where the particles have a distribution of two different size ranges.
While hydrophobic or super-hydrophobic materials have been described above, there remains a need for hydrophobic or super-hydrophobic materials in cosmetic compositions to impart superhydrophobic films on surfaces such as skin, hair, or nails. Conventional water-proof or water-resistant cosmetic compositions are generally made from oil-in-water or water-in-oil emulsions. Water-in-oil emulsions tend to have an oily feel, thus limiting their use. The conventional approach to formulating water-proof or water-resistant cosmetic compositions relies on the use of hydrophobic film formers (e.g. waxes) to form a water-resistant barrier. Such conventional cosmetics are at best hydrophobic, as opposed to the super-hydrophobic films of the present invention.
Conventional water-proof or water-resistant topical compositions are not super-hydrophobic primarily because they lack nano-scale or micro-scale surface roughness. In the absence of roughness on the nano- or micro-meter scale, smooth films made of currently known hydrophobic materials exhibit contact angles that are not in the super-hydrophobic range, i.e., they are less than 140°. It would be desirable to provide cosmetic films which impart super-hydrophobic films for improving water repellency, self-cleaning properties, and long-wear properties.
It is therefore an object of the invention to provide cosmetic compositions for application to the skin, hair, or nails which form a super-hydrophobic film thereon. It is a further object of the invention to provide methods for imparting superhydrophobic films to skin, hair, and nails to achieve water-resistant, self-cleaning and/or long-wear properties.